Implement single character write interface

This basic interface just moves the cursor to the next position after
writing a character and supports CR and LF as special chars for now.

framebuffer.vhd

@@ -16,7 +16,11 @@ entity framebuffer is
         x: in std_logic_vector(9 downto 0);
         y: in std_logic_vector(8 downto 0);
 
-        rgb: out std_logic_vector(23 downto 0)
+        rgb: out std_logic_vector(23 downto 0);
+
+        write_enable: std_logic;
+        write_address: std_logic_vector(12 downto 0);
+        write_data: std_logic_vector(7 downto 0)
     );
 end framebuffer;
 
@@ -41,6 +45,7 @@ architecture logic of framebuffer is
 
     constant font: rom_type := read_font("font.hex");
 
+    signal read_address: std_logic_vector(12 downto 0);
     signal current_char: std_logic_vector(7 downto 0);
 
     signal current_glyph: std_logic_vector(63 downto 0);
@@ -49,52 +54,17 @@ architecture logic of framebuffer is
     constant glyph_pos_length: integer := 2;
     type glyph_pos_type is array(1 to glyph_pos_length) of integer range 0 to 63;
     signal glyph_pos: glyph_pos_type;
-
-    signal write_x: unsigned(6 downto 0);
-    signal write_y: unsigned(5 downto 0);
-    signal write_value: std_logic_vector(7 downto 0);
-
-    signal ra: std_logic_vector(12 downto 0);
-    signal wa: std_logic_vector(12 downto 0);
 begin
 
-    ra <= x(9 downto 3) & y(8 downto 3);
-    wa <= std_logic_vector(write_x) & std_logic_vector(write_y);
-
-    write_value <= "0" & std_logic_vector(write_x + 32);
-
-    cycle_write_location:
-    process(clk)
-        constant max_delay: integer := 2000000;
-        variable delay: integer range 0 to max_delay;
-    begin
-        if rising_edge(clk) then
-            if delay = max_delay then
-                delay := 0;
-
-                if write_x = 79 then
-                    write_x <= (others => '0');
-                    if write_y = 59 then
-                        write_y <= (others => '0');
-                    else
-                        write_y <= write_y + 1;
-                    end if;
-                else
-                    write_x <= write_x + 1;
-                end if;
-            else
-                delay := delay + 1;
-            end if;
-        end if;
-    end process cycle_write_location;
+    read_address <= x(9 downto 3) & y(8 downto 3);
 
     terminal_buffer: entity work.terminal_buffer port map (
         clk => clk,
-        ra => ra,
+        ra => read_address,
         do => current_char,
-        we => '1',
-        wa => wa,
-        di => write_value
+        we => write_enable,
+        wa => write_address,
+        di => write_data
     );
 
     process(clk)

main.vhd

@@ -51,6 +51,9 @@ begin
         clk => clk_vga,
         reset => reset,
 
+        write_enable => '0',
+        write_data => x"00",
+
         dvi_d => dvi_d,
         dvi_clk_p => dvi_clk_p,
         dvi_clk_n => dvi_clk_n,

terminal.vhd

@@ -15,6 +15,9 @@ entity terminal is
         clk: in std_logic;
         reset: in std_logic;
 
+        write_enable: in std_logic;
+        write_data: in std_logic_vector(7 downto 0);
+
         dvi_d: out std_logic_vector(11 downto 0);
         dvi_clk_p: out std_logic;
         dvi_clk_n: out std_logic;
@@ -31,8 +34,56 @@ architecture syn of terminal is
     signal image_x: std_logic_vector(9 downto 0);
     signal image_y: std_logic_vector(8 downto 0);
     signal pixel_rgb: std_logic_vector(23 downto 0);
+
+    signal fb_write_enable: std_logic;
+    signal fb_write_address: std_logic_vector(12 downto 0);
+    signal fb_write_data: std_logic_vector(7 downto 0);
+
+    signal write_x: unsigned(6 downto 0);
+    signal write_y: unsigned(5 downto 0);
 begin
 
+    fb_write_address <= std_logic_vector(write_x) & std_logic_vector(write_y);
+
+    process(clk)
+        variable next_line: unsigned(5 downto 0);
+    begin
+        if rising_edge(clk) then
+            fb_write_enable <= '0';
+            fb_write_data <= write_data;
+
+            -- calculate next line
+            if write_y = 59 then
+                next_line := (others => '0');
+            else
+                next_line := write_y + 1;
+            end if;
+
+            if write_enable = '1' then
+                fb_write_enable <= '1';
+                if write_x = 79 then
+                    write_x <= (others => '0');
+                    write_y <= next_line;
+                else
+                    write_x <= write_x + 1;
+                end if;
+
+                -- carriage return
+                if write_data = x"0d" then
+                    fb_write_enable <= '0';
+                    write_x <= (others => '0');
+                end if;
+
+                -- line feed (implicit CR)
+                if write_data = x"0a" then
+                    fb_write_enable <= '0';
+                    write_x <= (others => '0');
+                    write_y <= next_line;
+                end if;
+            end if;
+        end if;
+    end process;
+
     dvi_clk_ds: obufds port map (
         I => clk,
         O => dvi_clk_p,
@@ -68,7 +119,10 @@ begin
         clk => clk,
         x => image_x,
         y => image_y,
-        rgb => pixel_rgb
+        rgb => pixel_rgb,
+        write_enable => fb_write_enable,
+        write_address => fb_write_address,
+        write_data => fb_write_data
     );
 
 end syn;